1. Field of the Invention
This invention relates to an auxiliary exposure device for use with an image forming apparatus that forms the image on the recording medium by a slit-scanning exposure system.
2. Description of Related Art
An auxiliary exposure device is used for irradiating a secondary light onto a photosensitive recording medium, a photoreceptor or the like when the original image has an intermediate color portion, such as found in a photograph, is to be copied onto the receiving medium by the image forming apparatus. When the copied image is formed by a primary exposing light, irradiated onto the original image through a slit and then reflected to the photosensitive recording, or receiving, medium, the light and shade of the intermediate color portion of the copied image is accentuated (the contrast is accentuated in monochrome copy).
Conversely, if the photosensitive recording medium is exposed to a secondary light source at the same time, the light and shade of the formed image is softened and the tone reproduction or tone gradation is improved. This is especially true for the reproduced images of the middle tones (for instance, gray). These middle tones are output more clearly and approach the tone of the original image, such as a photograph.
Prior art devices addressing this problem normally have a secondary light source, used only for the auxiliary exposure, in addition to the primary light source providing for the main exposure of the original. The secondary light source providing the auxiliary exposure was installed in or near the exposure portion of the image forming apparatus where the photosensitive recording medium is irradiated by the reflected light from the original. The light intensity of the secondary light source irradiated onto the recording medium is usually about 20-30 percent of the amount of light received from the primary light source.
However, in these conventional image forming apparatuses, the color temperature of the secondary light source might be different from that of the primary light source because the primary light source providing the main exposure is different from the secondary light source providing the auxiliary exposure. Therefore, the color of the formed image may be changed from that of the original, that is, the copy has its color image shifted to appear more reddish or bluish. Further, adjusting the amount of exposure by the secondary light source has been very difficult as it usually has been adjusted by changing the voltage applied to the secondary light source. Further, the construction of the device is complicated and results in high manufacturing costs as the primary and secondary source lights are separately installed.
Image forming apparatus that address the abovenoted problems are known. One such image forming apparatus is disclosed in Japanese Utility Model Laid-Open Application No. 64(1989)-17532 (published Jan. 27, 1989). In this image forming apparatus as disclosed, a movable reflection board is provided between a light source and the original image. To form a copy image with this apparatus, the light is irradiated onto the original without simultaneously irradiating the reflection board. Next, the light is irradiated onto the reflection board by placing the reflection board between the light source and the original image. The light reflected by the reflection board has the same effect as an auxiliary exposure in the dual light apparatus. Thus, it is unnecessary to install a secondary light source to be used only for the auxiliary exposure and the problem with the conventional apparatus is solved. However, it is now necessary to have two exposure processes in order to form the image.
A second such image forming apparatus is disclosed in Japanese Utility Model Laid-Open Application No. 64(1989)-23044 (Published Feb. 7, 1989). In this apparatus, a slit-scanning exposure system is used with a reflection board, that reflects a part of the light emitted from the light source, provided between the original image and the slit. When a copy image is formed using this apparatus, both the main exposure and the auxiliary exposure are executed at the same time, because a part of the light from the light source irradiating to the original also irradiates to the reflection board.
With this second image forming apparatus, the image is formed using one exposure process and the problem with the conventional apparatuses is solved. However, the degree of the auxiliary exposure cannot be changed.